Liquid crystal display device

ABSTRACT

A small-sized active matrix type liquid crystal display device that may achieve large-sized display, high precision, high resolution and multi-gray scales is provided. Gray scale display is performed by combining time ratio gray scale and voltage gray scale in a liquid crystal display device which performs display in OCB mode. In doing so, one frame is divided into subframes corresponding to the number of bit for the time ratio gray scale. Initialize voltage is applied onto the liquid crystal upon display of a subframe.

This application is a continuation of U.S. application Ser. No.09/534,812, filed on Mar. 24, 2000 now U.S. Pat. No. 7,145,536.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device, morespecifically, a display device in which gray scale display is made byboth the voltage gray scale method and the time ratio gray scale.

2. Description of the Related Art

A technique that has recently accomplished rapid development is tomanufacture a semiconductor device in which semiconductor thin films areformed on an inexpensive glass substrate, for example, a thin filmtransistor (TFT). This rapid development is caused by a growing demandfor active matrix type display devices.

In an active matrix display device, a pixel TFT is placed in each ofpixel regions as many as several hundred thousands to several millionsarranged in matrix, and electric charge that flows into and out of apixel electrode connected to each pixel TFT is controlled by theswitching function of the pixel TFT.

As images are displayed with higher definition and higher resolution,demand for multi-gray scale display, desirably, in full color, has beenestablished in recent years.

Accompanying the movement regarding display devices towards higherdefinition and higher resolution, the active matrix display device thathas drawn attention most is a digital driven active matrix displaydevice that can be driven at a high speed.

The digital driven active matrix display device needs a D/A convertercircuit (DAC) for converting digital video data inputted from theexternal into analogue data (voltage gray scale). There are variouskinds of D/A converter circuits.

The multi-gray scale display capability of the digital driver activematrix display device is dependent on the capacity of this D/A convertercircuit, namely, how many bits of digital video data the D/A convertercircuit can convert into analogue data. For instance, in general, adisplay device having a D/A converter circuit that processes 2 bitdigital video data is capable of 2²=4 gray scale display. If the circuitprocesses 8 bit data, the device is capable of 2⁸=256 gray scaledisplay, if n bit, 2^(n) gray scale display.

However, enhancement of the capacity of the D/A converter circuit costscomplicated circuit structure and enlarged layout area for the D/Aconverter circuit. According to a lately reported display device, a D/Aconverter circuit is formed on the same substrate where an active matrixcircuit is formed, using a poly-silicon TFT. In this case, the structureof the D/A converter circuit is complicated to lower the yield of theD/A converter circuit, resulting in yield decrease of the displaydevice. In addition, increased layout area of the D/A converter circuitmakes it difficult to downsize the display device.

Further, a problem arose in the response speed of liquid crystalmolecules in a conventionally well-known TN mode (twist nematic mode)which uses nematic liquid crystal, as the time for writing an image dataonto a pixel became shorter, due to large sized display, high precisionand high resolution of an active matrix liquid crystal display device.

As described above, materialization of an active matrix liquid crystaldisplay device which achieves large sized display, high precision, highresolution and multi gray scale has been desired.

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems above and,the present invention provides a liquid crystal display device thatachieves large sized display, high precision, high resolution and multigray scale.

First, reference is made to FIG. 1. FIG. 1 is a structural diagramschematically showing a liquid crystal display device of the presentinvention. Reference numeral 101 denotes a liquid crystal display panelcomprising digital drivers. Liquid crystal display panel 101 comprisesan active matrix substrate 101-1 and an opposing substrate 101-2. Anactive matrix substrate 101-1 comprises a source driver 101-1-1, a gatedriver 101-1-2, and 101-1-3, and an active matrix circuit 101-1-4 inwhich a plurality of pixel TFTs are disposed in a matrix. The sourcedriver 101-1-1 and the gate drivers 101-1-2 and 101-1-3 drive the activematrix circuit 101-1-4. An opposing substrate 101-2 comprises anopposing electrode 401-2-1. Further, a terminal COM denotes a terminalwhich supplies signal to the opposing electrode.

Reference numeral 102 denotes a digital video data time ratio gray scaleprocessing circuit. The digital video data time ratio gray scaleprocessing circuit 102 converts, among m bit digital video data inputtedfrom the external, n bit digital video data into n bit digital videodata for voltage gray scale. Gray scale information of (m−n) bit data ofthe m bit digital video data is expressed by time ratio gray scale.

The n bit digital video data converted by the digital video data timeratio gray scale processing circuit 102 is inputted to the display panel101. The n bit digital video data inputted to the display panel 101 isthen inputted to the source driver 101-1-1 and converted into analoguegray scale data by the D/A converter circuit within the source driverand sent to each source signal line, then sent to pixel TFTs.

Reference numeral 103 denotes an opposing electrode driving circuit,which sends an opposing electrode control signal for controlling theelectric potential of an opposing electrode to an opposing electrode101-2-1 of the liquid crystal panel 101.

Note that through the specification, a liquid crystal display device anda liquid crystal panel are discriminated from each other. One that hasat least an active matrix circuit is referred to as a liquid crystalpanel.

Here, a description is made on a structural diagram schematicallyshowing a liquid crystal panel in a liquid crystal display device of thepresent invention by referring to FIGS. 2 and 3. Those that comprise theliquid crystal panel 101, namely an active matrix substrate 101-1, anopposing substrate and liquid crystal 101-3 are shown in FIGS. 2 and 3.The liquid crystal panel used in the present invention has a so-called“π cell structure”, and uses a display mode called OCB (opticallycompensated bend) mode. In the π cell structure, liquid crystalmolecules are aligned such that pre-tilt angles of the molecules aresymmetrical with respect to the center plane between the active matrixsubstrate and the opposing substrate. The orientation in the π cellstructure is splay orientation when the voltage is not applied to thesubstrates, and shifts into bend orientation shown in FIG. 2 when thevoltage is applied. Further application of voltage brings liquid crystalmolecules in bend orientation to an orientation perpendicular to thesubstrates, which allows light to transmit therethrough.

As shown in FIG. 2, a liquid crystal display panel of the presentinvention comprises a liquid crystal panel in which liquid crystal is inbend orientation, a biaxial phase difference plate 111 and a pair ofpolarizing plates whose transmission axes are perpendicular to eachother. In the OCB mode display, visual angle dependency of retardationis three-dimensionally compensated using biaxial phase differenceplates.

Liquid crystal molecules are in splay orientation shown in FIG. 3 whenthe voltage is not applied to the liquid crystal, as mentioned above.

Using the OCB mode, a high-speed response about ten times faster thanthat of the conventional TN mode may be realized.

Another example of the liquid crystal display device of the presentinvention is shown in FIG. 30. Reference numeral 301 denotes a liquidcrystal display device comprising analogue drivers. The liquid crystaldisplay device 301 comprises an active matrix substrate 301-1 and anopposing substrate 301-2. The active matrix substrate 301-1 is comprisedof a source driver 301-1-1, gate drivers 301-1-2, 301-1-3, an activematrix circuit 301-1-4 with a plurality of pixel TFTs arranged inmatrix. The source driver 301-1-1 and the gate drivers 301-1-2, 301-1-3drive the active matrix circuit 301-1-4. The opposing substrate 301-2has an opposing electrode 301-2-1. A terminal COM is a terminal forsupplying the opposing electrode with a signal.

Reference numeral 302 denotes an A/D converter circuit that convertsanalogue video data sent from the external into m bit digital videodata. Reference numeral 303 denotes a digital video data time ratio grayscale processing circuit. The digital video data time ratio gray scaleprocessing circuit 303 converts, of inputted m bit digital video data, nbit digital video data into n bit digital video data for voltage grayscale method. Gray scale information of (m−n) bit data of the inputted mbit digital video data is expressed in time ratio gray scale. The n bitdigital video data converted by the digital video data time ratio grayscale processing circuit 303 is inputted to a D/A converter circuit 304and then converted into analogue video data. The analogue video dataconverted by the D/A converter circuit 304 is inputted to the liquidcrystal display device 301. The analogue video data inputted to theliquid crystal display device 301 is then inputted to the source driverand sampled by a sampling circuit within the source driver so as to besent to each source signal line and to pixel. TFTs.

Denoted by 305 is an opposing electrode driving circuit, which sends anopposing electrode control signal for controlling the electric potentialof the opposing electrode to the opposing electrode 301-2-1 of theliquid crystal display device 301.

Details of the operation of the liquid crystal display device of thepresent invention will be described in Embodiment modes below.

A description is given on the structure of the present invention below.

According to the present invention, there is provided a liquid crystaldisplay device comprising:

An active matrix substrate comprising an active matrix circuit thatcomprises a plurality of pixel TFTs arranged in matrix, and a sourcedriver and a gate driver for driving the active matrix circuit; and

an opposing substrate having an opposing electrode, characterized inthat

display is made in the OCB mode, and in that,

of m bit digital video data inputted from the external, n bit data and(m−n) bit data are used as voltage gray scale information and time ratiogray scale information, respectively, (m and n are both positiveintegers equal to or larger than 2 and satisfy m>n), to thereby conductthe voltage gray scale and the time gray scale, simultaneously.

According to the present invention, there is provided a liquid crystaldisplay device comprising:

an active matrix substrate comprising an active matrix circuit thatcomprises a plurality of pixel TFTs arranged in matrix and a sourcedriver and a gate driver for driving the active matrix circuit; and

an opposing substrate having an opposing electrode, characterized inthat

display is made in the OCB mode, and in that,

of m bit digital video data inputted from the external, n bit data and(m−n) bit data are used as voltage gray scale information and time ratiogray scale information, respectively, (m and n are both positiveintegers equal to or larger than 2 and satisfy m>n), to thereby conductfirst the voltage gray scale and then the time ratio gray scale, orconduct one immediately before conducting the other.

According to the present invention, there is provided a liquid crystaldisplay device comprising:

an active matrix substrate comprising an active matrix circuit thatcomprises a plurality of pixel TFTs arranged in matrix and a sourcedriver and a gate driver for driving the active matrix circuit;

an opposing substrate having an opposing electrode; and

a circuit for converting m bit digital video data inputted from theexternal into n bit digital video data, and for supplying the sourcedriver with the n bit digital video data (m and n are both positiveintegers equal to or larger than 2, and satisfy m>n), characterized inthat

display is made by conducting simultaneously the voltage gray scale andthe time ratio gray scale, and by forming one frame of image from2^(m−n) sub-frames, and in that

voltage is applied to change the orientation of liquid crystal moleculesinto bend orientation upon starting to display the 2^(m−n) sub-frames.

According to the present invention, there is provided a liquid crystaldisplay device comprising:

an active matrix substrate comprising an active matrix circuit thatcomprises a plurality of pixel TFTs arranged in matrix and a sourcedriver and a gate drives for driving the active matrix circuit;

an opposing substrate comprising an opposing electrode; and

a circuit for converting m bit digital video data inputted from theexternal into n bit digital video data, and for supplying the sourcedriver with the n bit digital video data (m and n are both positiveintegers equal to or larger than 2, and satisfy m>n), characterized inthat

the voltage gray scale is first conducted to conduct and next the timeratio gray scale or one is conducted immediately before the other, andin that,

voltage is applied to change the orientation of liquid crystal moleculesinto bend orientation upon starting to display the 2^(m−n) sub-frames.

According to the present invention, there is provided a liquid crystaldisplay device comprising:

an active matrix substrate comprising an active matrix circuit thatcomprises a plurality of pixel TFTs arranged in matrix, and a sourcedriver and a gate driver for driving the active matrix circuit;

an opposing substrate having an opposing electrode; and

a circuit for converting m bit digital video data inputted from theexternal into n bit digital video data, and for supplying the sourcedriver with the n bit digital video data (m and n are both positiveintegers equal to or larger than 2, and satisfy m>n), characterized inthat

display is made by conducting simultaneously the voltage gray scale andthe time ratio gray scale, and by forming one frame of image from2^(m−n) sub-frames, and in that

voltage is applied to change the orientation of liquid crystal moleculesinto bend orientation upon starting to display a frame that is comprisedof the 2^(m−n) sub-frames.

According to the present invention, there is provided a liquid crystaldisplay device comprising:

an active matrix substrate comprising an active matrix circuit thatcomprises a plurality of pixel TFTs arranged in matrix, and a sourcedriver and a gate driver for driving the active matrix circuit;

an opposing substrate which comprises an opposing electrode; and

a circuit for converting m bit digital video data inputted from theexternal into n bit digital video data, and for supplying the sourcedriver with the n bit digital video data (m and n are both positiveintegers equal to or larger than 2, and satisfy m>n), characterized inthat

the voltage gray scale is first conducted and next the time ratio grayscale, or one is conducted immediately before the other, and in that

voltage is applied to change the orientation of liquid crystal moleculesinto bend orientation upon starting to display a frame that is comprisedof 2^(m−n) sub-frames.

The above-mentioned m and n may be 10 and 2, respectively.

The above-mentioned m and n may be 12 and 4, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an outlined structural diagram of a liquid crystal displaydevice of the present invention.

FIG. 2 is an outlined structural diagram of a liquid crystal panel ofthe present invention.

FIG. 3 is an outlined structural diagram of a liquid crystal panel ofthe present invention.

FIG. 4 is an outlined structural diagram of a liquid crystal displaydevice of the present invention.

FIG. 5 is a circuit structure diagram of an active matrix circuit, asource driver and gate drivers according to an embodiment mode of aliquid crystal display device of the present invention.

FIG. 6 is a diagram showing gray scale display levels according to anembodiment mode of a liquid crystal display device of the presentinvention.

FIG. 7 is a diagram showing a driving timing chart according to anembodiment mode of a liquid crystal display device of the presentinvention.

FIG. 8 is a diagram showing a driving timing chart according to anembodiment mode of a liquid crystal display device of the presentinvention.

FIG. 9 is a diagram showing a driving timing chart according to anembodiment mode of a liquid crystal display device of the presentinvention.

FIG. 10 is a diagram showing a driving timing chart according to anembodiment mode of a liquid crystal display device of the presentinvention.

FIG. 11 is a diagram showing a driving timing chart according to anembodiment mode of a liquid crystal display device of the presentinvention.

FIG. 12 is an outlined structural diagram of a liquid crystal displaydevice of the present invention.

FIG. 13 is a circuit structure diagram of an active matrix circuit, asource driver and gate drivers according to an embodiment mode of aliquid crystal display device of the present invention.

FIGS. 14A to 14C are diagrams showing an example of manufacturingprocesses for a liquid crystal display device of the present invention.

FIGS. 15A to 15C are diagrams showing an example of manufacturingprocesses for a liquid crystal display device of the present invention.

FIGS. 16A to 16C are diagrams showing an example of manufacturingprocesses for a liquid crystal display device of the present invention.

FIGS. 17A to 17C are diagrams showing an example of manufacturingprocesses for a liquid crystal display device of the present invention.

FIGS. 18A to 18C are diagrams showing an example of manufacturingprocesses for a liquid crystal display device of the present invention.

FIGS. 19A to 19C are diagrams showing an example of manufacturingprocesses for a liquid crystal display device of the present invention.

FIG. 20 is a diagram showing cross sectional structure of a displaydevice according to the present invention.

FIG. 21 is a structural diagram schematically showing a 3-plate typeprojector using a liquid crystal display device of the presentinvention.

FIG. 22 is a structural diagram schematically showing a 3-plate typeprojector using a liquid crystal display device of the presentinvention.

FIG. 23 is a structural diagram schematically showing a single platetype projector using a liquid crystal display device of the presentinvention.

FIGS. 24A and 24B are structural diagrams schematically showing a fronttype projector and a rear type projector using a liquid crystal displaydevice of the present invention.

FIG. 25 is a structural diagram schematically showing goggle typedisplay using a liquid crystal display device of the present invention.

FIG. 26 is a timing chart of field sequential driving.

FIG. 27 is a structural diagram schematically showing a notebook typepersonal computer using a liquid crystal display device of the presentinvention.

FIGS. 28A to 28D show examples of an electronic device using a liquidcrystal display device of the present invention.

FIGS. 29A to 29D show examples of an electronic device using a liquidcrystal display device of the present invention.

FIG. 30 is an outlined structural diagram of a liquid crystal displaydevice of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A liquid crystal display device of the present invention will now bedescribed in detail using preferred embodiment modes. However, theliquid crystal display device of the present invention is not limited tothe embodiment modes below.

Embodiment Mode 1

FIG. 4 schematically shows a structural diagram of a liquid crystaldisplay device of this embodiment mode. In this embodiment mode, aliquid crystal display device to which 4 bit digital video data is sentfrom the external is taken as an example with the intention ofsimplifying the explanation.

Shown in FIG. 4 is a schematic structural diagram of a liquid crystaldisplay device according to the present invention. Reference numeral 401denotes a liquid crystal panel having digital drivers. The liquidcrystal panel 401 comprises an active matrix substrate 401-1 and anopposing substrate 401-2. The active matrix substrate 401-1 is comprisedof a source driver 401-1-1, gate drivers 401-1-2 and 401-1-3, and anactive matrix circuit 401-1-4 with a plurality of pixel TFTs arranged inmatrix. The source driver 401-1-1 and the gate drivers 401-1-2 and401-1-3 drive the active matrix circuit 401-1-4. The opposing substrate401-2 has an opposing electrode 401-2-1. A terminal COM is a terminalfor supplying the opposing electrode with a signal.

The liquid crystal panel of this embodiment mode adopts the OCB modementioned above as its display mode.

Reference numeral 402 denotes a digital video data time ratio gray scaleprocessing circuit. The digital video data time ratio gray scaleprocessing circuit 402 converts, of 4 bit digital video data inputtedfrom the external, 2 bit digital video data into 2 bit digital videodata for voltage gray scale. Gray scale information of the other 2 bitdigital video data out of the 4 bit digital video data is expressed intime ratio gray scale.

The 2 bit digital video data underwent the conversion by the digitalvideo data time ratio gray scale processing circuit 402 is inputted tothe liquid crystal panel 401. The 2 bit digital video data inputted tothe liquid crystal panel 401 is then inputted to the source driver andconverted into analogue gray scale data by a D/A converter circuit (notshown) within the source driver so as to be sent to each source signalline.

Reference numeral 403 denotes an opposing electrode driving circuit,which sends an opposing electrode control signal for controlling theelectric potential of the opposing electrode to the opposing electrode401-2-1 of the liquid crystal panel 401.

Here, a description is given with reference to FIG. 5 of the circuitstructure for the liquid crystal panel 401 of the liquid crystal displaydevice according to this embodiment mode, in particular, the activematrix circuit 401-1-4.

The active matrix circuit 401-1-4 has (x×y) pieces of pixels in thisembodiment mode. For convenience's sake in explanation, each pixel isdesignated by a symbol such as P1,1, P2,1, . . . and Py,x. Also, eachpixel has a pixel TFT 501 and a storage capacitor 503. Liquid crystal isheld between the active matrix substrate and the opposing substrate.Liquid crystal 502 schematically shows the liquid crystal for each ofthe pixel.

The digital driver liquid crystal panel of this embodiment mode performsso-called line sequential driving in which pixels on one line (e.g.,P1,1, P1,2, . . . , P1,x) are driven simultaneously. In other words,analogue gray scale voltage is written into one line of pixels at once.A time required to write analogue gray scale voltage in all the pixels(P1,1 to Py,x) is named here one frame term (Tf). One frame term (Tf) isdivided into four terms, which are referred to as sub-frame terms (Tsf)in this embodiment mode. Further, a time required to write analogue grayscale voltage in one line of pixels (e.g., P1,1, P1,2, . . . , P1,x) iscalled one sub-frame line term (Tsfl).

The opposing electrode 401-2-1 receives an opposing electrode controlsignal sent from the opposing electrode control circuit. Specifically,the opposing electrode control signal is sent to the terminal COM towhich the opposing electrode is electrically connected.

Gray scale display in the liquid crystal display device of thisembodiment mode will next be described. The digital video data sent fromthe external to the liquid crystal display device of this embodimentmode is 4 bit and contains information of 16 gray scales. Here,reference is made to FIG. 6. FIG. 6 shows display gray scale levels ofthe liquid crystal display device of this embodiment mode. The voltagelevel VL is the lowest voltage level of voltages inputted to the D/Aconverter circuit. The voltage level VH is the highest voltage level ofvoltages inputted to the D/A converter circuit.

In this embodiment mode, the level between the voltage level VH and thevoltage level VL is divided equally into four to obtain voltage level of2 bit, namely, of 4 gray scale, and each step of the voltage level isdesignated α Here, a is: (α=(VH−VL)/4). Therefore, the voltage grayscale level outputted from the D/A converter circuit of this embodimentmode is VL when the address of the digital video data is (00). VL+α whenthe address of the digital video data is (01), VL+2α when the address ofthe digital video data is (10), and VL+3α when the address of thedigital video data is (11).

The D/A converter circuit of this embodiment mode can output fourpatterns of gray scale voltage levels, namely VL, (VL+α), (VL+2α) and(VL+3α), as described above. Then combining them with the time ratiogray scale display, the present invention may increase the number ofdisplay gray scale levels for the liquid crystal display device.

In this embodiment mode, information contained in 2 bit digital videodata of the 4 bit digital video data is used for the time ratio grayscale display to obtain more finely divided, or increased display grayscale levels where one voltage gray scale level α is further dividedequally into four levels. That is, the liquid crystal display device ofthis embodiment may acquire display gray scale levels corresponding tovoltage gray scale levels of VL, VL+α/4, VL+2α/4, VL+3α/4, VL+α,VL+5α/4, VL+6α/4, VL+7α/4, VL+2α, VL+9α/4, VL+10α/4, VL+11α/4 and VL+3α.

The 4 bit digital video data address inputted from the external; timeratio gray scale processed digital video data address and correspondingvoltage gray scale level; and display gray scale level combined with thetime gray scale are related in the following Table 1.

TABLE 1 Gray Scale Digital Time Ratio Gray Scale processed Display LevelVideo Address of Digital Video Data combined with Data (Gray ScaleVoltage Level) Time Ratio Gray Address 1st Tsfl 2nd Tsfl 3rd Tsfl 4thTsfl Scale 00 00 00 (VL) 00 (VL) 00 (VL) 00 (VL) VL 01 00 (VL) 00 (VL)00 (VL) 01 (VL + α) VL + α/4 10 00 (VL) 00 (VL) 01 (VL + α) 01 (VL + α)VL + 2α/4 11 00 (VL) 01 (VL + α) 01 (VL + α) 01 (VL + α) VL + 3α/4 01 0001 (VL + α) 01 (VL + α) 01 (VL + α) 01 (VL + α) VL + α 01 01 (VL + α) 01(VL + α) 01 (VL + α) 10 (VL + 2α) VL + 5α/4 10 01 (VL + α) 01 (VL + α)10 (VL + 2α) 10 (VL + 2α) VL + 6α/4 11 01 (VL + α) 10 (VL + 2α) 10 (VL +2α) 10 (VL + 2α) VL + 7α/4 10 00 10 (VL + 2α) 10 (VL + 2α) 10 (VL + 2α)10 (VL + 2α) VL + 2α 01 10 (VL + 2α) 10 (VL + 2α) 10 (VL + 2α) 11 (VL +3α) VL + 9α/4 10 10 (VL + 2α) 10 (VL + 2α) 11 (VL + 3α) 11 (VL + 3α)VL + 10α/4 11 10 (VL + 2α) 11 (VL + 3α) 11 (VL + 3α) 11 (VL + 3α) VL +11α/4 11 00 11 (VL + 3α) 11 (VL + 3α) 11 (VL + 3α) 11 (VL + 3α) VL + 3α01 11 (VL + 3α) 11 (VL + 3α) 11 (VL + 3α) 11 (VL + 3α) VL + 3α 10 11(VL + 3α) 11 (VL + 3α) 11 (VL + 3α) 11 (VL + 3α) VL + 3α 11 11 (VL + 3α)11 (VL + 3α) 11 (VL + 3α) 11 (VL + 3α) VL + 3α

As shown in Table 1, the same gray scale voltage level of (VL+3α) isoutputted when the address of the 4 bit digital video data is (1100) to(1111).

Incidentally, the gray scale voltage levels shown in Table 1 may be thevoltages actually applied to the liquid crystal. In other words, a grayscale voltage level shown in Table 1 may be of a voltage leveldetermined by taking into consideration V_(COM) applied to the opposingelectrode which will be described later.

The liquid crystal display device of this embodiment carries out displayby dividing one frame term Tf into four sub-frame terms (1st Tsf, 2ndTsf, 3rd Tsf and 4th Tsf). Since the line sequential driving isconducted in the liquid crystal display device of this embodiment mode,gray scale voltage is written in each pixel during one sub-frame lineterm (Tsfl). Therefore, during sub-frame line terms (1st Tsfl, 2nd Tsfl,3rd Tsfl and 4th Tsfl) corresponding to the sub-frame terms (1st Tsf,2nd Tsf, 3rd Tsf and 4th Tsf), the address of time-gray scale processed2 bit digital video data is inputted to the D/A converter circuit, whichthen outputs gray scale voltages. With the gray scale voltage writtenduring the four sub-frame line terms (1st Tsfl, 2nd Tsfl, 3rd Tsfl and4th Tsfl), four sub-frames are displayed at a high speed. As a result,one frame of display gray scale corresponds to a value obtained bytotaling the gray scale voltage levels in the sub-frame line terms andthen time-averaging the total. The voltage gray scale and the time ratiogray scale are thus simultaneously conducted.

In the liquid crystal display device of this embodiment mode, aninitialize term (Ti) is provided prior to the start of the sub-frameline term in each sub-frame term. During this initialize term (Ti), acertain voltage Vi (pixel electrode initialize voltage) is applied toall the pixels and a certain voltage V_(COMi) (opposing electrodeinitialize voltage) is applied to the opposing electrode, whereby theliquid crystal in splay orientation shifts into bend orientation.

Thus the display of 2⁴−3=13 gray scale levels can be obtained in theliquid crystal display device of this embodiment mode even in case ofusing the D/A converter circuit handling 2 bit digital video data.

The addresses (or gray scale voltage levels) of the digital video datawritten during the sub-frame line terms (1st Tsfl, 2nd Tsfl, 3rd Tsfl,and 4th Tsfl) may be set using a combination other than the combinationsshown in Table 1. For instance, in Table 1, a gray scale voltage of(VL+α) is written during the third sub-frame line term (3rd Tsfl) andthe fourth sub-frame line term (4th Tsfl), when the digital video dataaddress is (0010). However, the present invention can be carried outwithout being limited to this combination. This means that the digitalvideo data whose address is (0010) merely requires (VL+α) gray scalevoltage to be written during any two sub-frame line terms out of foursub-frame line terms, i.e., the first sub-frame line term to the fourthsub-frame line term. There is no limitation in choosing and settingthose two sub-frame line terms during which (VL+α) gray scale voltage isto be written.

Now, reference is made to FIGS. 7 and 8. FIGS. 7 and 8 together show adrive timing chart for the liquid crystal display device of thisembodiment mode. The pixel P1.1, the pixel P2,1, the pixel P3,1 and thepixel Py,1 are taken as an example and shown in FIGS. 7 and 8. The drivetiming chart is divided and shown in two diagrams, i.e., FIGS. 7 and 8.because of limited spaces.

As described above, one frame term (Tf) consists of the first sub-frameterm (1st Tsf), the second sub-frame term (2nd Tsf), the third sub-frameterm (3rd Tsf), and the fourth sub-frame term (4th Tsf). The initializeterm (Ti) is placed before every sub-frame term, and a pixel electrodeinitialize voltage (V_(i)) is applied to all the pixels during thisinitialize term (Ti). An opposing electrode initialize voltage(V_(COMi)) is also applied to the opposing electrode (COM) during theinitialize term (Ti).

Therefore, in this embodiment mode, a voltage of (V_(i)+V_(COMi)) isapplied to the liquid crystal sandwiched between the pixel electrode andthe opposing electrode during the initialize term (Ti). This voltageapplication brings the liquid crystal molecules in splay orientationinto bend orientation, so that the device reaches the state where ahigh-speed response is possible also with later application of analoguegray scale voltage having image information.

Digital video data is converted by the D/A converter circuit intoanalogue gray scale voltage and is written in the pixel P1,1 during thefirst sub-frame line term (1 st Tsfl) subsequent to passing of theinitialize term (Ti). After the initialize term (Ti), V_(COM) is appliedto the opposing electrode. Incidentally, V_(COM) can be adjusted inaccordance with the degree of flicker on the display screen. V_(COM) maybe 0 V.

It is desirable to set optimal values for V_(i), V_(COMi), and V_(COM)in consistent with liquid crystal to be used and the quality of display.

After digital video data is converted by the D/A converter circuit intoanalogue gray scale voltage and written in the pixels P1,1 to P1,x,during the next sub-frame line term, the D/A converter circuit convertsdigital video data into analogue gray scale voltage and the voltage iswritten in the pixels P2,1 to P2,x.

In this way, the analogue gray scale voltage having image information iswritten in order in all the pixels, completing the first sub-frame term.

Subsequent to the first sub-frame term, the second sub-frame term isstarted. In the second sub-frame term (2 nd Tsf) also, the opposingelectrode (COM) is supplied with the opposing electrode initializevoltage (V_(COMi)) during the initialize term (Ti). And after theinitialize term (Ti) is passed, digital video data is converted by theD/A converter circuit into analogue gray scale voltage and written inthe pixels P1,1 to P1,x during the second sub-frame line term (2ndTsfl). After digital video data is converted by the D/A convertercircuit into analogue gray scale voltage and written in the pixels P1,1to P1,x, during the next sub-frame line term, the D/A converter circuitconverts digital video data into analogue gray scale voltage and thevoltage is written in the pixels P2,1 to P2,x. Application of V_(COM) tothe opposing electrode follows the passing of the initialize term (Ti).

In this way, the analogue gray scale voltage having image information iswritten in order in all the pixels, completing the second sub-frameterm.

Similar operation is carried out during the third sub-frame term (3rdTsf) and the fourth sub-frame term (4th Tsf).

The first sub-frame term (1st Tsf) to the fourth sub-frame term (4thTsf) are thus completed.

Subsequent to the completion of the first frame term, the second frameterm is started (FIG. 8). This embodiment mode includes carrying out theframe inversion in which direction of the electric field applied to theliquid crystal is alternately inverted as one frame term ends and thenext frame term begins. Therefore in the second frame term, the pixelelectrode initialize voltage (Vi) and the gray scale voltages which areto be supplied to the pixel electrode has the opposite polarity to theone in the first frame term, by taking the opposing electrode as thereference electric potential.

Here, reference is made to FIG. 9. FIG. 9 exemplarily shows therelationship between the gray scale voltage level written in the pixelelectrode of a certain pixel (pixel P1,1, for example) for everysub-frame term and gray scale display level during the frame term.

Firstly reference is made to the first frame term. The initializevoltage (V_(i)) is first applied to the pixel electrode during theinitialize term (Ti), so that the liquid crystal in splay orientationshifts into bend orientation. After the initialize term (Ti) is ended, agray scale voltage of (VL+α) is written during the first sub-frame lineterm (1st Tsfl) and gray scale display corresponding to the gray scalevoltage of (VL+α) is conducted during the first sub-frame term (1stTsf). Then, a gray scale voltage of (VL+2α) is written during the secondsub-frame line term (2nd Tsfl) and gray scale display corresponding tothe gray scale voltage of (VL+2α) is conducted during the secondsub-frame term (2nd Tsf). Subsequently, a gray scale voltage of (VL+2α)is written during the third sub-frame line term (3rd Tsfl) and grayscale display corresponding to the gray scale voltage of (VL+2α) isconducted during the third sub-frame term (3rd Tsf). Thereafter, a grayscale voltage of (VL+2α) is written during the fourth sub-frame lineterm (4th Tsfl) and gray scale display corresponding to the gray scalevoltage of (VL+2α) is conducted during the fourth sub-frame term (4thTsf). The gray scale display level in the first frame, therefore,corresponds to the gray scale voltage level of (VL+7α/4).

Turning next to the second frame term, the initialize voltage (V_(i)) isfirst applied to the pixel electrode during the initialize term (Ti), sothat the liquid crystal in splay orientation shifts into bendorientation. After the initialize term (Ti) is ended, a gray scalevoltage of (VL+2α) is written during the first sub-frame line term (1stTsfl) and gray scale display corresponding to the gray scale voltage of(VL+2α) is conducted during the first sub-frame term (1st Tsf). Then, agray scale voltage of (VL+2α) is written during the second sub-frameline term (2nd Tsfl) and gray scale display corresponding to the grayscale voltage of (VL+2α) is conducted during the second sub-frame term(2nd Tsf). Subsequently, a gray scale voltage of (VL+3α) is writtenduring the third sub-frame line term (3rd Tsfl) and gray scale displaycorresponding to the gray scale voltage of (VL+3α) is conducted duringthe third sub-frame term (3rd Tsf). Thereafter, a gray scale voltage of(VL+3α) is written during the fourth sub-frame line term (4th Tsfl) andgray scale display corresponding to the gray scale voltage of (VL+3α) isconducted during the fourth sub-frame term (4th Tsf). The gray scaledisplay level in the second frame, therefore, corresponds to the grayscale voltage level of (VL+10α/4).

In this embodiment mode, in order to obtain the voltage level of fourgray scales, the level between the voltage level VH and the voltagelevel VL is divided equally into four each having the value α. However,the present invention is still effective if the level between thevoltage level VH and the voltage level VL is not divided equally butirregularly.

The gray scale voltage levels are realized by, in this embodiment mode,inputting the voltage level VH and the voltage level VL into the D/Aconverter circuit of the liquid crystal panel. This may be accomplishedby inputting a voltage level of 3 or more, instead.

Though the gray scale voltage level written during the sub-frame lineterms is set as shown in Table 1 in this embodiment mode, as mentionedabove, it is not limited to the values in Table 1.

In this embodiment, of the 4 bit digital video data inputted from theexternal, 2 bit digital video data is converted into 2 bit digital videodata for voltage gray scale and gray scale information of another 2 bitdigital video data of the 4 bit digital video data is expressed in timeratio gray scale. Now, consider a general example where n bit digitalvideo data of m bit digital video data from the external is convertedinto digital video data for gray scale voltage by a time ratio grayscale processing circuit while gray scale information of (m−n) bit datathereof is expressed in time ratio gray scale. The symbol m and n areboth integers equal to or larger than 2 and satisfy m>n.

In this case, the relationship between frame term (Tf) and sub-frameterm (Tsf) is expressed as follows:Tf=2^(m−n) ·TsfTherefore, (2^(m)−(2^(m−n)−1)) patterns of gray scale display isobtained.

This embodiment mode takes as an example the case where m=4 and n=2.Needless to say, the present invention is not limited to that example.The symbols m and n may take 12 and 4, respectively, or 8 and 2. It isalso possible to set m to 8 and n to 6, or to 10 and to 2. Values otherthan those may be used as well.

The voltage gray scale and the time gray scale may be conducted in theorder stated, or one after another continuously.

Embodiment Mode 2

This embodiment mode gives a description of a case where frame inversiondriving is carried out for every sub-frame in the liquid crystal displaydevice of the present invention which has the structure shown inEmbodiment Mode 1.

Reference is made to FIG. 10. FIG. 10 shows a drive timing chart for theliquid crystal display device of this embodiment mode. The pixel P1,1,the pixel P2,1, the pixel P3,1 and the pixel Py,1 are taken as anexample and shown in FIG. 10.

In this embodiment mode also, as described above, one frame term (Tf)consists of the first sub-frame term (1st Tsf), the second sub-frameterm (2nd Tsf), the third sub-frame term (3rd Tsf), and the fourthsub-frame term (4th Tsf). The initialize term (Ti) is placed beforeevery sub-frame term, and the pixel electrode initialize voltage (V_(i))is applied to all the pixels during this initialize term (Ti). Anopposing electrode initialize voltage (V_(COMi)) is also applied to theopposing electrode (COM) during the initialize term (Ti).

Therefore, in this embodiment also, a voltage of (V_(i)+V_(COMi)) isapplied to the liquid crystal sandwiched between the pixel electrode andthe opposing electrode during the initialize term (Ti). This voltageapplication brings the liquid crystal molecules in splay orientationinto bend orientation, so that the device reaches the state where ahigh-speed response is possible even in case of later applying analoguegray scale voltage having image information.

In the first sub-frame term, after passing an initialize term (Ti),digital video data is converted by the D/A converter circuit intoanalogue gray scale voltage and the analogue gray scale voltage iswritten in the pixel P1,1 during the first sub-frame line term (1stTsfl). In the pixels P1,1 to P1,x, analogue gray scale voltagecorresponding to each pixel is written simultaneously. Note here thatafter the initialize term (Ti), V_(COM) is applied to the opposingelectrode. Incidentally, V_(COM) can be adjusted in accordance with thedegree of flicker on the display screen. This embodiment mode may take 0V for V_(COM).

After digital video data is converted by the D/A converter circuit intoanalogue gray scale voltage and written in the pixels P1,1 to P1,x,during the next sub-frame line term, the D/A converter circuit convertsdigital video data into analogue gray scale voltage and the voltage iswritten in the pixels P2,1 to P2,x.

In this way, the analogue gray scale voltage having image information iswritten in order in all the pixels, completing the first sub-frame term.

Subsequent to the first sub-frame term, the second sub-frame term isstarted. In the second sub-frame term (2nd Tsf) also, the opposingelectrode (COM) is supplied with the opposing electrode initializevoltage (V_(COMi)) during the initialize term (Ti). Note that theelectric field to be applied to the liquid crystal is inverted inpolarity for every subframe, in this embodiment mode. It is the same inthe second sub-frame term as in the first sub-frame term that, after theinitialize term (Ti) is passed, digital video data is converted by theD/A converter circuit into analogue gray scale voltage and written inthe pixels P1,1 to P1,x during the first sub-frame line term (1st Tsfl).After digital video data is converted by the D/A converter circuit intoanalogue gray scale voltage and written in the pixels P1,1 to P1,x,during the next sub-frame line term, the D/A converter circuit convertsdigital video data into analogue gray scale voltage and the voltage iswritten in the pixels P2,1 to P2,x. Application of V_(COM) to theopposing electrode follows the passing of the initialize term (Ti).

In this way, the analogue gray scale voltage having image information iswritten in order in all the pixels, completing the second sub-frameterm.

Similar operation is carried out during the third sub-frame term (3rdTsf) and the fourth sub-frame term (4th Tsf).

The first sub-frame term (1st Tsf) to the fourth sub-frame term (4thTsf) are thus completed.

Subsequent to the completion of the first frame term, the second frameterm is started (not shown).

As seen in the above, display in this embodiment mode employs sub-frameinversion system in which direction of the electric field applied to theliquid crystal is inverted every time a sub-frame is ended to start thenext one, to thereby obtain less flickering display.

Embodiment 3

This embodiment mode employs the structure explained in Embodiment Mode1 for the liquid crystal display device of the present invention. Adescription given here is about a case where only the first sub-frameterm has the initialize term so that the initialize voltage (V_(i) andV_(COM)) are applied and the frame inversion driving is conducted.

Reference is made to FIG. 11. FIG. 11 shows a drive timing chart for theliquid crystal display device of this embodiment mode. The pixel P1,1,the pixel P2,1, the pixel P3,1 and the pixel Py,1 are taken as anexample and shown in FIG. 11.

In this embodiment mode also, as described above, one frame term (Tf)consists of the first sub-frame term (1st Tsf), the second sub-frameterm (2nd Tsf), the third sub-frame term (3rd Tsf), and the fourthsub-frame term (4th Tsf). The difference of this embodiment mode fromEmbodiment Mode 1 resides in that the initialize term (Ti) is placedbefore the start of the first sub-frame term only, to apply the pixelelectrode initialize voltage (V_(i)) to all the pixels during thisinitialize term (Ti).

That the opposing electrode initialize voltage (V_(COMi)) is applied tothe opposing electrode (COM) during the initialize term (Ti) is the sameas Embodiment Mode 1.

Therefore, in this embodiment mode also, a voltage of (V_(i)+V_(COMi))is applied to the liquid crystal sandwiched between the pixel electrodeand the opposing electrode during the initialize term (Ti). This voltageapplication brings the liquid crystal molecules from splay orientationinto bend orientation, so that the device reaches the state where ahigh-speed response is possible in case of later applying analogue grayscale voltage having image information.

In the first sub-frame term, digital video data is converted by the D/Aconverter circuit into analogue gray scale voltage and the analogue grayscale voltage is written in the pixel 1,1 during the first sub-frameline term (1st Tsfl) subsequent to passing of the initialize term (Ti).In the pixels P1,1 to P1,x, analogue gray scale voltage corresponding toeach pixel is written simultaneously. After the initialize term (Ti),V_(COM) is applied to the opposing electrode. Incidentally, V_(COM) canbe adjusted in accordance with the degree of flicker on the displayscreen. This embodiment mode may take 0 V for V_(COM).

After digital video data is converted by the D/A converter circuit intoanalogue gray scale voltage and written in the pixels P1,1 to P1,x,during the next sub-frame line term, the D/A converter circuit convertsdigital video data into analogue gray scale voltage and the voltage iswritten in the pixels P2,1 to P2,x.

In this way, the analogue gray scale voltage having image information iswritten in order in all the pixels, completing the first sub-frame term.

Subsequent to the first sub-frame term, the second sub-frame term isstarted. The initialize term (Ti) is not provided in the secondsub-frame term (2nd Tsf). Accordingly, the initialize voltage (V_(i) andV_(COM)) are not applied to the pixels upon the start of the secondsub-frame term. Digital video data is converted by the D/A convertercircuit into analogue gray scale voltage and written in the pixels P1,1to P1,x during the first sub-frame line term (1st Tsfl). After digitalvideo data is converted by the D/A converter circuit into an analoguegray scale voltage and written in the pixels P1,1 to P1,x, during thenext sub-frame line term, the D/A converter circuit converts digitalvideo data into analogue gray scale voltage and the voltage is writtenin the pixels P2,1 to P2,x.

In this way, the analogue gray scale voltage having image information iswritten in order in all the pixels, completing the second sub-frameterm.

Operation similar to the one in the second sub-frame term (2nd Tsf) iscarried out during the third sub-frame term (3rd Tsf) and the fourthsub-frame term (4th Tsf).

The first sub-frame term to the fourth sub-frame term are thuscompleted.

Subsequent to the completion of the first frame term, the second frameterm is started (not shown).

Embodiment Mode 4

A description given in this embodiment deals with a liquid crystaldisplay device to which 10 bit digital video data is inputted. Referenceis made to FIG. 12 that schematically shows the structure of the liquidcrystal display device of this embodiment mode. Reference numeral 1001denotes a liquid crystal display device having an active matrixsubstrate 1001-1 and an opposing substrate 1001-2. The active matrixsubstrate 1001-1 comprises source drivers 1001-1-1 and 1001-1-2, a gatedriver 1001-1-3, an active matrix circuit 1001-1-4 with a plurality ofpixel TFTs arranged in matrix, a digital video data time ratio grayscale processing circuit 1001-1-5, and an opposing electrode drivingcircuit 1001-1-6. The opposing substrate 1001-2 has an opposingelectrode 1001-2-1. A terminal COM is a terminal for supplying theopposing electrode with a signal.

In this embodiment, as shown in FIG. 12, the digital video data timeratio gray scale processing circuit is integrally formed with theopposing electrode driving circuit on the active matrix substrate,forming as a whole the liquid crystal display device.

The digital video data time gray scale processing circuit 1001-1-5converts, of 10 bit digital video data inputted from the external, 8 bitdigital video data into 8 bit digital video data for voltage gray scale.Gray scale information of 2 bit digital video data of the 10 bit digitalvideo data is expressed in time gray scale.

The 8 bit digital video data converted by the digital video data timeratio gray scale processing circuit 1001-1-5 is inputted to the sourcedrivers 1001-1-1, 1001-1-2, converted into analogue gray scale voltageby D/A converter circuits (not shown) within the source drivers, andsent to each source signal line.

Now take a look at FIG. 13. FIG. 13 shows more detailed circuitstructure of the liquid crystal display device of this embodiment mode.The source driver 1001-1-1 comprises a shift register circuit1001-1-1-1, a latch circuit 1 (1001-1-1-2), a latch circuit 2(1001-1-1-3), and a D/A converter circuit (1001-1-1-4). Other thanthose, the source driver includes a buffer circuit and a level shiftercircuit (neither is shown). For the convenience in explanation, the D/Aconverter circuit 1001-1-1-4 assumedly includes a level shifter circuit.

The source driver 1001-1-2 has the same structure as that of the sourcedriver 1001-1-1. The source driver 1001-1-1 sends an image signal (grayscale voltage) to odd-numbered source signal lines and the source driver1001-1-2 sends an image signal to even-numbered source signal lines.

In the active matrix liquid crystal display device of this embodimentmode, to suit the convenience of the circuit layout, two source drivers1001-1-1, 1001-1-2 are arranged sandwiching vertically the active matrixcircuit. However, only one source driver may be used if that is possiblein view of the circuit layout.

The gate driver 1001-1-3 includes a shift register circuit, a buffercircuit, a level shifter circuit, etc., (all of which is not shown).

The active matrix circuit 1001-1-4 contains 1920 (in width)×1080 (inlength) pixels. Each pixel has the structure similar to the onedescribed in the above Embodiment 1.

The liquid crystal display device of this embodiment has the D/Aconverter circuit 1001-1-1-4 that processes 8 bit digital video data.Information contained in 2 bit data of 10 bit digital video datainputted from the external is used for time gray scale. The time grayscale here is the same as in the above Embodiment 1.

Therefore, the liquid crystal display device of this embodiment canobtain 2⁸−3=253 patterns of gray scale display.

The liquid crystal display device of this embodiment may be driven byany of the driving methods shown in the above Embodiment Modes 1 to 3.

Embodiment Mode 5

This embodiment mode describes an example of manufacturing method of aliquid crystal display device of the present invention. Explained hereis a method in which TFTs for an active matrix circuit and TFTs for adriver circuit arranged in the periphery of the active matrix circuitare formed at the same time.

[Step of Forming Island Semiconductor Layer and Gate Insulating Film:FIG. 14A]

In FIG. 14A, non-alkaline glass substrate or a quartz substrate ispreferably used for a substrate 7001. A silicon substrate or a metalsubstrate that have an insulating film formed on the surface, may alsobe used.

On one surface of the substrate 7001 on which the TFT is to be formed, abase film made of a silicon oxide film, a silicon nitride film, or asilicon nitride oxide film is formed by plasma CVD or sputtering to havea thickness of 100 to 400 nm. For instance, a base film 7002 may beformed with a two-layer structure in which a silicon nitride film 7002having a thickness of 25 to 100 nm, here in 50 nm, and a silicon oxidefilm 7003 having a thickness of 50 to 300 nm, here in 150 nm, areformed. The base film 7002 is provided for preventing impuritycontamination from the substrate, and is not always necessary if aquartz substrate is employed.

Next, an amorphous silicon film with a thickness of 20 to 100 nm isformed on the base film 7002 by a known film formation method. Thoughdepending on its hydrogen content, the amorphous silicon film ispreferably heated at 400 to 550° C. for several hours fordehydrogenation, reducing the hydrogen content to 5 atomic % or less toprepare for the crystallization step. The amorphous silicon film may beformed by other formation methods such as sputtering or evaporation. Inthis case, it is desirable that impurity elements such as oxygen andnitrogen etc. contained in the film be sufficiently reduced. The basefilm and the amorphous silicon film can be formed by the same filmformation method here, so that the films may be formed continuously. Inthat case, it is possible to prevent contamination on the surface sinceit is not exposed to the air, and that reduces fluctuation incharacteristics of the TFTs to be manufactured.

A known laser crystallization technique or thermal crystallizationtechnique may be used for a step of forming a crystalline silicon filmfrom the amorphous silicon film. The crystalline silicon film may beformed by thermal oxidation using a catalytic element for promoting thecrystallization of silicon. Other options include the use of amicrocrystal silicon film and direct deposition of a crystalline siliconfilm. Further, the crystalline silicon film may be formed by employing aknown technique of SOI (Silicon On Insulators) with which asingle-crystal silicon is adhered to a substrate.

An unnecessary portion of the thus formed crystalline silicon film isetched and removed to form island semiconductor layers 7004 to 7006. Aregion in the crystalline silicon film where an N channel TFT is to beformed may be doped in advance with boron (B) in a concentration ofabout 1×10¹⁵ to 5×10¹⁷ cm⁻³ in order to control the threshold voltage.

Then a gate insulating film 7007 comprising mainly silicon oxide orsilicon nitride is formed to cover the island semiconductor layers 7004to 7006. The thickness of the gate insulating film 7007 may be 10 to 200nm, preferably 50 to 150 nm. For example, the gate insulating film maybe fabricated by forming a silicon nitride oxide film by plasma CVD withraw materials of N₂O and SiH₄ in a thickness of 75 nm, and thenthermally oxidizing the film in an oxygen atmosphere or a mixedatmosphere of oxygen and hydrogen chloride at 800 to 1000° C. into athickness of 115 nm (FIG. 14A).

[Formation of N⁻ Region: FIG. 14B]

Resist masks 7008 to 7011 are formed over the entire surfaces of theisland semiconductor layers 7004 and 7006 and region where wiring is tobe formed, and over a portion of the island semiconductor layer 7005(including a region to be a channel formation region) and a lightlydoped region 7012 is formed by doping impurity element imparting n-type.This lightly doped region 7012 is an impurity region for forming lateran LDD region (called an Lov region in this specification, where ‘ov’stands for ‘overlap’) that overlaps with a gate electrode through thegate insulating film in the N channel TFT of a CMOS circuit. Theconcentration of the impurity element for imparting n type in thelightly doped region formed here is referred to as (n⁻). Accordingly,the lightly doped region 7012 may be called n⁻ region in thisspecification.

Phosphorus is doped by ion doping with the use of plasma-excitedphosphine (PH₃) without performing mass-separation on it. Needless tosay, the ion implantation involving mass-separation may be employedinstead. In this step, a semiconductor layer beneath the gate insulatingfilm 7007 is doped with phosphorus through the film 7007. Theconcentration of phosphorus to be used in the doping preferably rangesfrom 5×10¹⁷ atoms/cm³ to 5×10¹⁸ atoms/cm³, and the concentration here inthis embodiment mode is set to 1×10¹⁸ atoms/cm³.

Thereafter, the resist masks 7008 to 7011 are removed and heat treatmentis conducted in a nitrogen atmosphere at 400 to 900° C., preferably, 550to 800° C. for 1 to 12 hours, activating phosphorus added in this step.

[Formation of Conductive Films for Gate Electrode and for Wiring: FIG.14C]

A first conductive film 7013 with a thickness of 10 to 100 nm is formedfrom an element selected from tantalum (Ta), titanium (Ti), molybdenum(Mo) and tungsten (W) or from a conductive material comprising one ofthose elements as its main ingredient. Tantalum nitride (TaN) ortungsten nitride (WN), for example, is desirably used for the firstconductive film 7013. A second conductive film 7014 with a thickness of100 to 400 nm is further formed on the first conductive film 7013 froman element selected from Ta. Ti, Mo and W or from a conductive materialcomprising one of those elements as its main ingredient For instance, aTa film may be formed in a thickness of 200 nm. Though not shown, it iseffective to form a silicon film with a thickness of about 2 to 20 nmunder the first conductive film 7013 for the purpose of preventingoxidation of the conductive films 7013 or 7014 (especially theconductive film 7014).[Formation of P-ch Gate Electrode and Wiring Electrode, and Formation ofP⁺ Region: FIG. 15A]

Resist masks 7015 to 7018 are formed and the first conductive film andthe second conductive film (which are hereinafter treated as a laminatedfilm) are etched to form a gate electrode 7019 and gate wirings 7020 and7021 of a P channel TFT. Here, conductive films 7022 and 7023 are leftto cover the entire surface of the regions to be N channel TFTs.

Proceeding to the next step, the resist masks 7015 to 7018 are remainedas they are to serve as masks, and a part of the semiconductor layer7004 where the P channel TFT is to be formed is doped with an impurityelement for imparting p type. Boron may be used here as the impurityelement and is doped by ion doping (of course ion implantation may alsobe employed) using diborane (B₂H₆). Boron is doped here to aconcentration from 5×10²⁰ to 3×10²¹ atoms/cm³. The concentration of theimpurity element for imparting p type contained in the impurity regionsformed here is expressed as (p⁺⁺). Accordingly, impurity regions 7024and 7025 may be referred to as p⁺⁺regions in this specification.

Here, doping process of impurity element imparting p-type may beperformed instead after exposing a portion of island semiconductor layer7004 by removing gate insulating film 7007 by etching, using resistmasks 7015-7018. In this case, a low acceleration voltage is sufficientfor the doping, causing less damage on the island semiconductor film andimproving the throughput

[Formation of N-ch Gate Electrode: FIG. 15B]

Then the resist masks 7015 to 7018 are removed and new resist masks 7026to 7029 are formed to form gate electrodes 7030 and 7031 of the Nchannel TFTs. At this point, the gate electrode 7030 is formed so as tooverlap with the n⁻ region 7012 through the gate insulating film.

[Formation of N⁺ Region: FIG. 15C]

The resist masks 7026 to 7029 are then removed and new resist masks 7032to 7034 are formed. Subsequently, a step of forming an impurity regionfunctioning as a source region or a drain region in the N channel TFT iscarried out. The resist mask 7034 is formed so as to cover the gateelectrode 7031 of the N channel TFT. This is for forming in later stepan LDD region that do not overlap with the gate electrode in the Nchannel TFT of the active matrix circuit.

An impurity element imparting n type is added thereto to form impurityregions 7035 to 7039. Here, ion doping (of course ion implantation alsowill do) using phosphine (PH₃) is again employed, and the phosphorusconcentration in these regions are set to 1×10²⁰ to 1×10²¹ atoms/cm³.The concentration of the impurity element for imparting n type containedin the impurity regions 7037 to 7039 formed here is designated as (n⁻).Accordingly, the impurity regions 7037 to 7039 may be referred to as n⁺regions in this specification. The impurity regions 7035 and 7036 haven⁻ regions which have already been formed, so that, strictly speaking,they contain a slightly higher concentration of phosphorus than theimpurity regions 7037 to 7039 do.

Here, doping process of impurity element imparting n-type may beperformed instead after exposing a portion of island semiconductor layer7005 and 7006 by removing gate insulating film 7007 by etching usingresist masks 7032 to 7034 and gate electrode 7030 as masks. In thiscase, a low acceleration voltage is sufficient for the doping, causingless damage on the island-like semiconductor films and improving thethroughput.

[Formation of N⁻⁻ Region: FIG. 16A]

Next, the resist masks 7032 to 7034 are removed and an impurity elementimparting n type is doped in the island semiconductor layer 7006 wherethe N channel TFT of the active matrix circuit is to be formed. Thusformed impurity regions 7040 to 7043 are doped with phosphorus in thesame concentration as in the above n⁻ regions or a less concentration(specifically, 5×10¹⁶ to 1×10¹⁸ atoms/cm³). The concentration of theimpurity element imparting n type contained in the impurity regions 7040to 7043 formed here is expressed as (n⁻⁻). Accordingly, the impurityregions 7040 to 7043 may be referred to as n⁻⁻ regions in thisspecification. Incidentally, every impurity region except for animpurity region 7067 that is hidden under the gate electrode is dopedwith phosphorus in a concentration of n⁻⁻ in this step. However, thephosphorus concentration is so low that the influence thereof may beignored.

[Step of Thermal Activation: FIG. 16B]

Formed next is a protective insulating film 7044, which will laterbecome a part of a first interlayer insulating film. The protectiveinsulating film 7044 may comprise a silicon nitride film, a siliconoxide film, a silicon nitride oxide film or a laminated film combiningthose films. The film thickness thereof ranges from 100 nm to 400 nm.

Thereafter, a heat treatment step is carried out to activate theimpurity element added in the respective concentration for imparting ntype or p type. This step may employ the furnace annealing, the laserannealing or the rapid thermal annealing (RTA). Here in this embodimentmode, the activation step is carried out by the furnace annealing. Theheat treatment is conducted in a nitrogen atmosphere at 300 to 650° C.,preferably 400 to 550° C., in here 450° C., for 2 hours.

Further heat treatment is performed in an atmosphere containing 3 to100% of hydrogen at 300 to 450° C. for 1 to 12 hours, hydrogenating theisland semiconductor layer. This step is to terminate dangling bonds inthe semiconductor layer with thermally excited hydrogen. Otherhydrogenating means includes plasma hydrogenation (that uses hydrogenexcited by plasma).

[Formation of Interlayer Insulating Film, Source/drain Electrode,Light-shielding Film, Pixel Electrode and Holding Capacitance: FIG. 16C]

Upon completion of the activation step, an interlayer insulating film7045 with a thickness of 0.5 to 1.5 μm is formed on the protectiveinsulating film 7044. A laminated film consisting of the protectiveinsulating film 7044 and the interlayer insulating film 7045 serves as afirst interlayer insulating film.

After that, contact holes reaching to the source regions or the drainregions of the respective TFTs are formed to form source electrodes 7046to 7048 and drain electrodes 7049 and 7050. Though not shown, theseelectrodes in this embodiment mode comprise a laminated film having athree-layer structure in which a Ti film with a thickness of 100 nm, aTi-containing aluminum film with a thickness of 300 nm and another Tifilm with a thickness of 150 nm are sequentially formed by sputtering.

Then a passivation film 7051 is formed using a silicon nitride film, asilicon oxide film or a silicon nitride oxide film in a thickness of 50to 500 nm (typically, 200 to 300 nm). Subsequent hydrogenation treatmentperformed in this state brings a favorable result in regard to theimprovement of the TFT characteristics. For instance, it is sufficientif heat treatment is conducted in an atmosphere containing 3 to 100%hydrogen at 300 to 450° C. for 1 to 12 hours. The same result can beobtained when the plasma hydrogenation method is used. An opening may beformed here in the passivation film 7051 at a position where a contacthole is later formed for connecting pixel electrode and the drainelectrode.

Thereafter, a second interlayer insulating film 7052 made from anorganic resin is formed to have a thickness of about 1 μm. As theorganic resin, polyimide, acrylic, polyamide, polyimideamide, BCB(benzocyclobutene), etc. may be used. The advantages in the use of theorganic resin film include simple film formation, reduced parasiticcapacitance owing to low relative permittivity, excellent flatness, etc.Other organic resin films than the ones listed above or an organic-basedSiO compound may also be used. Here, polyimide of the type beingthermally polymerized after applied to the substrate is used and firedat 300° C. to form the film 7052.

Subsequently, a light-shielding film 7053 is formed on the secondinterlayer insulating film 7052 in area where active matrix circuit isformed. The light-shielding film 7053 comprises an element selected fromaluminum (Al), titanium (Ti) and tantalum (Ta) or of a film containingone of those elements as its main ingredient into a thickness of 100 to300 nm. On the surface of the light-shielding film 7053, an oxide film7054 with a thickness of 30 to 150 nm (preferably 50 to 75 nm) is formedby anodic oxidation or plasma oxidation. Here, an aluminum film or afilm mainly containing aluminum is used as the light-shielding film7053, and an aluminum oxide film (alumina film) is used as the oxidefilm 7054.

The insulating film is provided only on the surface of thelight-shielding film here in this embodiment mode. The insulating filmmay be formed by a vapor deposition method such as plasma CVD, thermalCVD, or by sputtering. In that case also, the film thickness thereof isappropriately 30 to 150 nm (preferably 50 to 75 nm). A silicon oxidefilm, a silicon nitride film, a silicon nitride oxide film, a DLC(Diamond-like carbon) film or an organic resin film may be used for theinsulating film. A lamination film with those films layered incombination may also be used.

Then a contact hole reaching the drain electrode 7050 is formed in thesecond interlayer insulating film 7052 to form a pixel electrode 7055.Note that pixel electrodes 7056 and 7057 are adjacent but individualpixels, respectively. For the pixel electrodes 7055 to 7057, atransparent conductive film may be used in the case of fabricating atransmission type display device and a metal film may be used in thecase of a reflection type display device. Here, in order to manufacturea transmission type display device, an indium tin oxide film (ITO) witha thickness of 100 nm is formed by sputtering.

At this point, a storage capacitor is formed in a region 7058 where thepixel electrode 7055 overlaps with the light-shielding film 7053 throughthe oxide film 7054.

In this way, an active matrix substrate having the CMOS circuit servingas a driver circuit and the active matrix circuit formed on the samesubstrate is completed. A P channel TFT 7081 and an N channel TFT 7082are formed in the CMOS circuit serving as a driver circuit, and a pixelTFT 7083 is formed from an N channel TFT in the active matrix circuit.

The P channel TFT 7081 of the CMOS circuit has a channel formationregion 7061, and a source region 7062 and a drain region 7063 formedrespectively in the p⁺ regions. The N channel TFT 7082 has a channelformation region 7064, a source region 7065, a drain region 7066 and anLDD region (hereinafter referred to as Lov region, where ‘ov’ stands for‘overlap’) 7067 that overlaps with the gate electrode through the gateinsulating film. The source region 7065 and the drain region 7066 areformed respectively in (n⁻+n⁺) regions and the Lov region 7067 is formedin the n⁻ region.

The pixel TFT 7083 has channel formation regions 7068 and 7069, a sourceregion 7070, a drain region 7071, LDD regions 7072 to 7075 which do notoverlap with the gate electrode through the gate insulating film(hereinafter referred to as Loff regions, where ‘off’ stands for‘offset’), and an n⁺ region 7076 in contact with the Loff regions 7073and 7074. The source region 7070 and the drain region 7071 are formedrespectively in the n⁺ regions and the Loff regions 7072 to 7075 areformed in the n⁻ regions.

According to the manufacturing process of the present embodiment mode,the structure of the TFTs for forming the active matrix circuit and forforming the driver circuit can be optimized in accordance with thecircuit specification each circuit requires, thereby improvingoperational performance and reliability of the semiconductor device. Inconcrete, by varying the arrangement of LDD regions of n-channel TFT byappropriately using Lov region or Loff region according to the circuitspecification, a TFT structure in which high operation or countermeasureto hot carrier is sought and a TFT structure in which low OFF currentoperation is sought are realized on the same substrate.

For instance, the N channel TFT 7082 is suitable for a logic circuitwhere importance is attached to the high speed operation, such as ashift register circuit, a frequency divider circuit, a signal dividingcircuit, a level shifter circuit and a buffer circuit. On the otherhand, the N channel TFT 7083 is suitable for a circuit where importanceis attached to the low OFF current operation, such as an active matrixcircuit and a sampling circuit (sample hold circuit).

The length (width) of the Lov region is 0.5 to 3.0 μm, typically 1.0 to1.5 μm, with respect to the channel length of 3 to 7 μm. The length(width) of the Loff regions 7072 to 7075 arranged in the pixel TFT 7083is 0.5 to 3.5 μm, typically 2.0 to 2.5 μm.

Through the above steps, an active matrix substrate is completed.

Next, a description will be given on a process of manufacturing a liquidcrystal display device using the active matrix substrate fabricatedthrough the above steps.

An alignment film (not shown) is formed on the active matrix substratein the state shown in FIG. 16C. In this embodiment mode, polyimide isused for the alignment film. An opposite substrate is then prepared. Theopposite substrate comprises a glass substrate, an opposing electrodemade of a transparent conductive film and an alignment film (neither ofwhich is shown).

A polyimide film is again used for the alignment film of the oppositesubstrate in this embodiment mode. After forming the alignment film,rubbing treatment is performed. The polyimide used for the alignmentfilm in this embodiment mode is one that has a relatively large pretiltangle.

The active matrix substrate and the opposite substrate which haveundergone the above steps are then adhered to each other by a known cellassembly process through a sealing material or a spacer (neither isshown). After that, liquid crystal is injected between the substratesand an end-sealing material (not shown) is used to completely seal thesubstrates. In this embodiment mode, nematic liquid crystal is used forthe injected liquid crystal.

A liquid crystal display device is thus completed.

Incidentally, the amorphous silicon film may be crystallized by laserlight (typically excimer laser light) instead of the crystallizationmethod for amorphous silicon film described in this embodiment mode.

Additionally, the polycrystalline silicon film may be replaced by an SOIstructure (SOI substrate) such as SmartCut™, a SIMOX, and ELTRAN™ toperform other processes.

Embodiment Mode 6

This embodiment mode gives a description on another manufacturing methodof a liquid crystal display device of the present invention. Thedescription here in this embodiment mode deals with a method ofsimultaneously manufacturing TFTs forming an active matrix circuit andthose forming a driver circuit arranged in the periphery of the activematrix circuit.

[Steps of Formation of Island-like Semiconductor Layer and GateInsulating Film: FIG. 17A]

In FIG. 17A, a non-alkaline glass substrate or a quartz substrate isdesirably used for a substrate 6001. A usable substrate other than thosemay be a silicon substrate or a metal substrate on the surface of whichan insulating film is formed.

On the surface of the substrate 6001 on which the TFT is to be formed, abase film 6002 made of a silicon oxide film, a silicon nitride film, ora silicon nitride oxide film is formed by plasma CVD or sputtering tohave a thickness of 100 to 400 nm. For instance, a base film 6002 ispreferably formed in a two-layer structure in which a silicon nitridefilm 6002 having a thickness of 25 to 100 nm, in here 50 nm, and asilicon oxide film 6003 having a thickness of 50 to 300 nm, in here 150nm, are layered. The base film 6002 is provided for preventing impuritycontamination from the substrate, and is not always necessary if aquartz substrate is employed.

Next, an amorphous silicon film with a thickness of 20 to 100 nm isformed on the base film 6002 by a known film formation method. Thoughdepending on its hydrogen content, the amorphous silicon film ispreferably heated at 400 to 550° C. for several hours fordehydrogenation, reducing the hydrogen content to 5 atomic % or less toprepare for the crystallization step. The amorphous silicon film may beformed by other formation methods such as sputtering or evaporation ifimpurity elements such as oxygen and nitrogen etc. contained in the filmare sufficiently reduced. The base film and the amorphous silicon filmcan be formed by the same film formation method here continuously. Inthat case, the device is not exposed to the air after forming the basefilm, which makes it possible to prevent contamination of the surfacereducing fluctuation in characteristics of the TFTs to be manufactured.

A known laser crystallization technique or thermal crystallizationtechnique may be used for a step of forming a crystalline silicon filmfrom the amorphous silicon film. The crystalline silicon film may beformed by thermal oxidation using a catalytic element for promoting thecrystallization of silicon. Other options include the use of amicrocrystal silicon film and direct deposition of a crystalline siliconfilm. Further, the crystalline silicon film may be formed by employing aknown technique of SOI (Silicon On Insulators) with which asingle-crystal silicon is adhered to a substrate.

An unnecessary portion of thus formed crystalline silicon film is etchedand removed to form island semiconductor layers 6004 to 6006. Boron maybe doped in advance in a region in the crystalline silicon film where anN channel TFT is to be formed in a concentration of about 1×10¹⁵ to5×10¹⁷ cm⁻³ in order to control the threshold voltage.

Then a gate insulating film 6007 containing mainly silicon oxide orsilicon nitride is formed to cover the island semiconductor layers 6004to 6006. The thickness of the gate insulating film 6007 is 10 to 200 nm,preferably 50 to 150 nm. For example, the gate insulating film may befabricated by forming a silicon nitride oxide film by plasma CVD withraw materials of N₂O and SiH₄ in a thickness of 75 nm, and thenthermally oxidizing the film in an oxygen atmosphere or a mixedatmosphere of oxygen and chlorine at 800 to 1000° C. into a thickness of115 nm (FIG. 17A).

[Formation of N⁻ Region: FIG. 17B]

Resist masks 6008 to 6011 are formed on the entire surfaces of theisland-like semiconductor layers 6004 and 6006 and region where a wiringis to be formed, and on a portion of the island semiconductor layer 6005(including a region to be a channel formation region) and lightly dopedregions 6012 and 6013 were formed by doping impurity element impartingn-type. These lightly doped regions 6012 and 6013 are impurity regionsfor later forming LDD regions that overlap with a gate electrode throughthe gate insulating film (called Lov regions in this specification,where ‘ov’ stands for ‘overlap ’) in the N channel TFT of a CMOScircuit. The concentration of the impurity element for imparting n typecontained in the lightly doped regions formed here is referred to as(n⁻). Accordingly, the lightly doped regions 6012 and 6013 may be calledn⁻ regions.

Phosphorus is doped by ion doping with the use of plasma-excitedphosphine (PH₃) without performing mass-separation on it. Of course, ionimplantation involving mass-separation may be employed instead. In thisstep, a semiconductor layer beneath the gate insulating film 6007 isdoped with phosphorus through the film 6007. The concentration ofphosphorus may preferably be set in a range from 5×10¹⁷ atoms/cm³ to5×10¹⁸ atoms/cm³, and the concentration here is set to 1×10¹⁸ atoms/cm³.

Thereafter, the resist masks 6008 to 6011 are removed and heat treatmentis conducted in a nitrogen atmosphere at 400 to 900° C., preferably 550to 800° C., for 1 to 12 hours, activating phosphorus added in this step.

[Formation of Conductive Films for Gate Electrode and for Wiring: FIG.17C]

A first conductive film 6014 with a thickness of 10 to 100 nm is formedfrom an element selected from tantalum (Ta), titanium (Ti), molybdenum(Mo) and tungsten (W) or from a conductive material containing one ofthose elements as its main ingredient. Tantalum nitride (TaN) ortungsten tungsten (WN), for example, is desirably used for the firstconductive film 6014. A second conductive film 6015 with a thickness of100 to 400 nm is further formed on the first conductive film 6014 froman element selected from Ta, Ti, Mo and W or from a conductive materialcontaining one of those elements as its main ingredient. For instance, ATa film is formed in a thickness of 200 nm. Though not shown, it iseffective to form a silicon film with a thickness of about 2 to 20 nmunder the first conductive film 6014 for the purpose of preventingoxidation of the conductive films 6014, 6015 (especially the conductivefilm 6015).

[Formation of P-ch Gate Electrode and Wiring Electrode, and Formation ofP⁺ Region: FIG. 18A]

Resist masks 6016 to 6019 are formed and the first conductive film andthe second conductive film (which are hereinafter treated as a laminatedfilm) are etched to form a gate electrode 6020 and gate wirings 6021 and6022 of a P channel TFT. Conductive films 6023, 6024 are left to coverthe entire surface of the regions to be N channel TFTs.

Proceeding to the next step, the resist masks 6016 to 6019 are remainedas they are to serve as masks, and a part of the semiconductor layer6004 where the P channel TFT is to be formed is doped with an impurityelement for imparting p type. Boron is selected here as the impurityelement and is doped by ion doping (of course ion implantation also willdo) using dibolane (B₂H₆). The concentration of boron used in the dopinghere is 5×10²⁰ to 3×10²¹ atoms/cm³. The concentration of the impurityelement for imparting p type contained in the impurity regions formedhere is expressed as (p⁺⁺). Accordingly, impurity regions 6025 and 6026may be referred to as p⁺⁺regions in this specification.

Here, doping process of impurity element imparting p-type may beperformed instead after exposing a portion of island semiconductor layer6004 by removing gate insulating film 6007 by etching using resist masks6016 to 6019. In this case, a low acceleration voltage is sufficient forthe doping, causing less damage on the island semiconductor film andimproving the throughput.

[Formation of N-ch Gate Electrode: FIG. 18B]

Then the resist masks 6016 to 6019 are removed and new resist masks 6027to 6030 are formed to form gate electrodes 6031 and 6032 of the Nchannel TFTs. At this point, the gate electrode 6031 is formed so as tooverlap with the n⁻ regions 6012. 6013 through the gate insulating film.

[Formation of N⁺ Region: FIG. 18C]

The resist masks 6027 to 6030 are then removed and new resist masks 6033to 6035 are formed. Subsequently, a step of forming an impurity regionfunctioning as a source region or a drain region in the N channel TFTwill be carried out. The resist mask 6035 is formed so as to cover thegate electrode 6032 of the N channel TFT. This is for forming in laterstep an LDD region which do not to overlap with the gate electrode inthe N channel TFT of the active matrix circuit.

An impurity element for imparting n type is added thereto to formimpurity regions 6036 to 6040. Here, ion doping (of course ionimplantation also will do) using phosphine (PH₃) is again employed, andthe phosphorus concentration in these regions is set to 1×10²⁰ to 1×10²¹atoms/cm³. The concentration of the impurity element contained in theimpurity regions 6038 to 6040 formed here is expressed as (n⁺).Accordingly, the impurity regions 6038 to 6040 may be referred to as n⁺regions in this specification. The impurity regions 6036, 6037 have n⁻regions which have already been formed, so that, strictly speaking, theycontain a slightly higher concentration of phosphorus than the impurityregions 6038 to 6040 do.

Here, doping process of impurity element imparting n-type may beperformed instead after exposing a portion of island semiconductor layer6005 and 6006 by removing gate insulating film 6007 by etching usingresist masks 6033 to 6035. In this case, a low acceleration voltage issufficient for the doping, causing less damage on the islandsemiconductor film and improving the throughput.

[Formation of N⁻⁻ region: FIG. 19A]

Next, a step is carried out in which the resist masks 6033 to 6035 areremoved and the island semiconductor layer 6006 where the N channel TFTof the active matrix circuit is to be formed is doped with an impurityelement for imparting n type. The thus formed impurity regions 6041 to6044 are doped with phosphorus in the same concentration as in the aboven⁻ regions or a less concentration (specifically, 5×10¹⁶ to 1×10¹⁸atoms/cm³). The concentration of the impurity element for imparting ntype contained in the impurity regions 6041 to 6044 formed here isexpressed as (n⁻⁻). Accordingly, the impurity regions 6041 to 6044 maybe referred to as n⁻⁻ regions in this specification. Incidentally, everyimpurity region except for an impurity region 6068 that is hidden underthe gate electrode is doped with phosphorus in a concentration of n⁻⁻ inthis step. However, the phosphorus concentration is so low that theinfluence thereof may be ignored.

[Step of Thermal Activation: FIG. 19B]

Formed next is a protective insulating film 6045, which will laterbecome a part of a first interlayer insulating film. The protectiveinsulating film 6045 may be made of a silicon nitride film, a siliconoxide film, a silicon nitride oxide film or a lamination film with thosefilms layered in combination. The film thickness thereof ranges from 100nm to 400 nm.

Thereafter, a heat treatment step is carried out to activate theimpurity elements added in the respective concentration for imparting ntype or p type. This step may employ the furnace annealing, the laserannealing or the rapid thermal annealing (RTA). Here, the activationstep is carried out by the furnace annealing. The heat treatment isconducted in a nitrogen atmosphere at 300 to 650° C., preferably 400 to550° C., in here 450° C., for 2 hours.

Further heat treatment is performed in an atmosphere containing 3 to100% of hydrogen at 300 to 450° C. for 1 to 12 hours, hydrogenating theisland semiconductor layer. This step is to terminate dangling bonds inthe semiconductor layer with thermally excited hydrogen. Otherhydrogenating means includes plasma hydrogenation (that uses hydrogenexcited by plasma).

[Formation of Interlayer Insulating Film, Source/drain Electrode,Light-shielding Film, Pixel Electrode and Holding Capacitance: FIG. 19C]

Upon completion of the activation step, an interlayer insulating film6046 with a thickness of 0.5 to 1.5 μm is formed on the protectiveinsulating film 6045. A lamination film consisting of the protectiveinsulating film 6045 and the interlayer insulating film 6046 serves as afirst interlayer insulating film.

After that, contact holes reaching to the source regions and the drainregions of the respective TFTs are formed to form source electrodes 6047to 6049 and drain electrodes 6050 and 6051. Though not shown, theseelectrodes in this embodiment mode are each made of a laminated filmhaving a three-layer structure in which a Ti film with a thickness of100 nm, a Ti-containing aluminum film with a thickness of 300 nm andanother Ti film with a thickness of 150 nm are sequentially formed bysputtering.

Then a passivation film 6052 is formed using a silicon nitride film, asilicon oxide film or a silicon nitride oxide film in a thickness of 50to 500 nm (typically, 200 to 300 nm). Subsequent hydrogenation treatmentperformed in this state brings a favorable result in regard to theimprovement of the TFT characteristics. For instance, it is sufficientif heat treatment is conducted in an atmosphere containing 3 to 100%hydrogen at 300 to 450° C. for 1 to 12 hours. The same result can beobtained when the plasma hydrogenation method is used. An opening may beformed here in the passivation film 6052 at a position where a contacthole for connecting the pixel electrode and the drain electrode is to beformed.

Thereafter, a second interlayer insulating film 6053 made from anorganic resin is formed to have a thickness of about 1 μm. As theorganic resin, polyimide, acrylic, polyamide, polyimideamide, BCB(benzocyclobutene), etc. may be used. The advantages in the use of theorganic resin film include simple film formation, reduced parasiticcapacitance owing to low relative permittivity, excellent flatness, etc.Other organic resin films than the ones listed above and anorganic-based SiO compound may also be used. Here, polyimide of the typebeing thermally polymerized after applied to the substrate is used andburst at 300° C. to form the film 6053.

Subsequently, a light-shielding film 6054 is formed on the secondinterlayer insulating film 6053 in a region to be the active matrixcircuit. The light-shielding film 6054 is made from an element selectedfrom aluminum (Al), titanium (Ti) and tantalum (Ta) or of a filmcontaining one of those elements as its main ingredient to have athickness of 100 to 300 nm. On the surface of the light-shielding film6054, an oxide film 6055 with a thickness of 30 to 150 nm (preferably 50to 75 nm) is formed by anodic oxidation or plasma oxidation. Here inthis embodiment mode, an aluminum film or a film mainly containingaluminum is used as the light-shielding film 6054, and an aluminum oxidefilm (alumina film) is used as the oxide film 6055.

The insulating film is provided only on the surface of thelight-shielding film here in this embodiment mode. The insulating filmmay be formed by a vapor phase method such as plasma CVD, thermal CVD orsputtering. In that case also, the film thickness thereof isappropriately 30 to 150 nm (preferably 50 to 75 nm). A silicon oxidefilm, a silicon nitride film, a silicon nitride oxide film, a DLC(Diamond like carbon) film or an organic resin film may be used for theinsulating film. A lamination film with those films layered incombination may also be used.

Then a contact hole reaching the drain electrode 6051 is formed in thesecond interlayer insulating film 6053 to form a pixel electrode 6056.Incidentally, pixel electrodes 6057 and 6058 are for adjacent butindividual pixels, respectively. For the pixel electrodes 6056 to 6058,a transparent conductive film may be used in the case of fabricating atransmission type display device and a metal film may be used in thecase of a reflection type display device. In the embodiment mode here,in order to manufacture a transmission type display device, an indiumtin oxide (ITO) film with a thickness of 100 nm is formed by sputtering.

At this point, a storage capacitor is formed using a region 6059 wherethe pixel electrode 6056 overlaps with the light-shielding film 6054through the oxide film 6055.

In this way, an active matrix substrate having the CMOS circuit servingas a driver circuit and the active matrix circuit which are formed onthe same substrate is completed. A P channel TFT 6081 and an N channelTFT 6082 are formed in the CMOS circuit serving as a driver circuit, anda pixel TFT 6083 is formed from an N channel TFT in the active matrixcircuit.

The P channel TFT 6081 of the CMOS circuit has a channel formationregion 6062, and a source region 6063 and a drain region 6064respectively formed in the p⁺regions. The N channel TFT 6082 has achannel formation region 6065, a source region 6066, a drain region 6067and LDD regions 6068 and 6069 which overlap with the gate electrodethrough the gate insulating film (hereinafter referred to as Lov region,where ‘ov’ stands for ‘overlap’). The source region 6066 and the drainregion 6067 are formed respectively in (n⁻+n⁺) regions and the Lovregion 6068 and 6069 are formed in the n⁻ region.

The pixel TFT 6083 has channel formation regions 6069 and 6070, a sourceregion 6071, a drain region 6072, LDD regions 6073 to 6076 which do notoverlap with the gate electrode through the gate insulating film(hereinafter referred to as Loff regions, where ‘off’ stands for‘offset’), and an n⁺ region 6077 in contact with the Loff regions 6074and 6075. The source region 6071 and the drain region 6072 are formedrespectively in the n⁺ regions and the Loff regions 6073 to 6076 areformed in the n⁻⁻ regions.

According to the manufacturing method of the present embodiment mode,the structure of the TFTs for forming the active matrix circuit and forforming the driver circuit can be optimized in accordance with thecircuit specification each circuit requires, thereby improvingoperational performance and reliability of the semiconductor device.Specifically, varying the arrangement of the LDD region in the N channelTFT and choosing either the Lov region or the Loff region in accordancewith the circuit specification realize formation on the same substrateof the TFT structure that attaches importance to high speed operation orto countermeasures for hot carrier and the TFT structure that attachesimportance to low OFF current operation.

For instance, in the case of the active matrix display device, the Nchannel TFT 6082 is suitable for a logic circuit where importance isattached to the high speed operation, such as a shift register circuit,a frequency divider circuit, a signal dividing circuit, a level shiftercircuit and a buffer circuit. On the other hand, the N channel TFT 6083is suitable for a circuit where importance is attached to the low OFFcurrent operation, such as an active matrix circuit and a samplingcircuit (sample hold circuit).

The length (width) of the Lov region is 0.5 to 3.0 μm, typically 1.0 to1.5 μm, with respect to the channel length of 3 to 7 μm. The length(width) of the Loff regions 6073 to 6076 arranged in the pixel TFT 6083is 0.5 to 3.5 μm, typically 2.0 to 2.5 μm.

A display device is manufactured using as the base the active matrixsubstrate fabricated through the above steps. For an example of themanufacturing process, see Embodiment mode 5.

Embodiment Mode 7

FIG. 20 shows an example of another structure of the active matrixsubstrate for the liquid crystal display device of the presentinvention. Reference numeral 8001 denotes a P channel TFT, while 8002,8003 and 8004 denote N channel TFTs. The TFTs 8001, 8002, 8003constitute a circuit portion of a driver, and 8004 is a component of anactive matrix circuit portion.

Reference numerals 8005 to 8013 denote semiconductor layers of the pixelTFT constituting the active matrix circuit. Denoted by 8005, 8009 and8013 are n⁺ regions; 8006, 8008, 8010 and 8012, n⁻⁻ regions; and 8007and 8011, channel formation regions. A cap layer of an insulating filmis designated by 8014, which is provided to form offset portions in thechannel formation regions.

As concerns this embodiment mode, see a patent application by thepresent applicant, Japanese Patent Application No. 11-67809.

Embodiment Mode 8

The display device of the present invention described above may be usedfor a three panel type projector as shown in FIG. 21.

In FIG. 21, reference numeral 2401 denotes a white light source; 2402 to2405, dichroic mirrors; 2406 and 2407, total reflection mirrors; 2408 to2410, display devices of the present invention; and 2411, a projectionlens.

Embodiment Mode 9

The display device of the present invention described above may be usedalso for a three panel type projector as shown in FIG. 22.

In FIG. 22, reference numeral 2501 denotes a white light source; 2502and 2503, dichroic mirrors; 2504 to 2506, total reflection mirrors; 2507to 2509, display devices of the present invention; 2510, a dichroicprism; and 2511, a projection lens.

Embodiment Mode 10

The liquid crystal display device of the present invention describedabove may be used also for a single panel type projector as shown inFIG. 23.

In FIG. 23, reference numeral 2601 denotes a white light sourcecomprising a lamp and a reflector, and 2602, 2603 and 2604 denotedichroic mirrors which selectively reflect light in wavelength regionsof blue, red and green, respectively. Denoted by 2605 is a microlensarray consisting of a plurality of microlenses. Reference numeral 2606denotes a liquid crystal display panel of the present invention; 2607, afield lens; 2608, a projection lens; and 2609, a screen.

Embodiment Mode 11

The projectors in Embodiment modes 8 to 10 above are classified intorear projectors and front projectors depending on their manner ofprojection.

FIG. 24A shows a front projector comprised of a main body 10001, adisplay device 10002 of the present invention, a light source 10003, anoptical system 10004, and a screen 10005. Though shown in FIG. 24A isthe front projector incorporating one display device, it may incorporatethree display devices (corresponding to the light R, G and B,respectively) to realize a front projector of higher resolution andhigher definition.

FIG. 24B shows a rear projector comprised of a main body 10006, adisplay device 10007, a light source 10008, a reflector 10009, and ascreen 10010. Shown in FIG. 24B is a rear projector incorporating threeactive matrix semiconductor display devices (corresponding to the lightR, G and B, respectively).

Embodiment Mode 12

This embodiment mode shows an example in which the display device of thepresent invention is applied to a goggle type display.

Reference is made to FIG. 25. Denoted by 2801 is the main body of agoggle type display; 2802-R, 2802-L, display devices of the presentinvention; 2803-R, 2803-L, LED backlights; and 2804-R, 2804-L, opticalelements.

Embodiment Mode 13

In this embodiment mode, LEDs are used for a backlight of a displaydevice of the present invention to perform a field sequential operation.

The timing chart of the field sequential driving method in FIG. 26 showsa start signal for writing a video signal (Vsync signal), lightingtiming signals (R, G and B) for red (R), green (G) and blue (B) LEDs,and a video signal (VIDEO). Tf indicates a frame period. Tr, Tg, Tbdesignate lit-up periods for red (R), green (G) and blue (B) LEDs,respectively.

A video signal sent to the display device, for example, R1, is a signalobtained by compressing along the time-base the video data, that isinputted from the external and corresponds to red, to have a size onethird the original data size. A video signal sent to the display panel,G1, is a signal obtained by compressing along the time-base the videodata, that is inputted from the external and corresponds to green, tohave a size one third the original data size. A video signal sent to thedisplay panel, B1, is a signal obtained by compressing along thetime-base the video data, that is inputted from the external andcorresponds to blue, to have a size one third the original data size.

In the field sequential driving method, R, G and B LEDs are litrespectively and sequentially during the LED lit-up periods: TR period,TG period and TB period. A video signal (R1) corresponding to red issent to the display panel during the lit-up period for the red LED (TR),to write one screen of red image into the display panel. A video data(G1) corresponding to green is sent to the display panel during thelit-up period for the green LED (TG), to write one screen of green imageinto the display panel. A video data (B1) corresponding to blue is sentto the display device during the lit-up period for the blue LED (TB), towrite one screen of blue image into the display device. These threetimes operations of writing images complete one frame of image.

Embodiment Mode 14

This embodiment mode shows with reference to FIG. 27 an example in whicha display device of the present invention is applied to a notebookcomputer.

Reference numeral 3001 denotes the main body of a notebook computer, and3002 denotes a display device of the present invention. LEDs are usedfor a backlight. The backlight may instead employ a cathode ray tube asin the prior art.

Embodiment Mode 15

The liquid crystal display device of the present invention may beapplied in various uses. In the present embodiment mode, semiconductordevices loading a display device of the present invention is explained.

Such semiconductor device include video camera, still camera, carnavigation systems, personal computer, portable information terminal(mobile computer, mobile telephone etc.). Examples of those are shown inFIG. 28.

FIG. 28A is a mobile telephone, comprising: main body 11001; voiceoutput section 11002; voice input section 11003; display device of thepresent invention 11004; operation switch 11005 and antenna 11006.

FIG. 28B shows a video camera comprising a main body 12001, a displaydevice 12002 of the present invention, an audio input unit 12003,operation switches 12004, a battery 12005, and an image receiving unit12006.

FIG. 28C shows a mobile computer comprising a main body 13001, a cameraunit 13002, an image receiving unit 13003, an operation switch 13004,and a display device 13005 of the present invention.

FIG. 28D shows a portable book (electronic book) comprising a main body14001, display devices 14002, 14003 of the present invention, storingmedium, operation switches 14005, and antenna 14006.

FIG. 29A is a personal computer, and comprises a main body 15001, imageinput section 15002, display section 15003 and key board 15004. Thepresent invention may be applied to an image input section 15002,display section 15003 and other signal control circuits.

FIG. 29B is a player using a recording medium in which a program isrecorded (hereinafter referred to as recording medium), and comprises amain body 16001, display section 16002, a speaker section 16003, arecording medium 16004 and operation switches 16005. By using DVD(digital versatile disc), CD, etc. for a recording medium, musicappreciation, film appreciation, game, or use for Internet may beperformed with this player. The present invention may be applied to thedisplay section 16002 and other signal control circuits.

FIG. 29C is a digital camera, and comprises a main body 17001, a displaysection 17002, a view finder 17003, an operation switch 17004 and imagereceiving section (not shown in the figure). The present invention maybe applied to the display section 17002 and other signal controlcircuits.

FIG. 29D is a display, and comprises a main body 18001, a supportingsection 18002 and a display section 18003. The present invention may beapplied to the display section 18003. The display of the presentinvention is specifically advantageous when the display is large-sized,and it is advantageous in a display of diagonal greater than 10 inches(more specifically a display of diagonal greater than 30 inches).

According to the present invention, an active matrix liquid crystaldisplay device having large-sized display, high precision, highresolution and multi-gray scales is realized.

1. A liquid crystal display device comprising: a first thin filmtransistor and a second thin film transistor, both of them in a pixel;and third thin film transistors in a source driver and a gate driver; anoptically compensated birefringence mode liquid crystal over the firstthin film transistor and the second thin film transistor; wherein eachof the first thin film transistor and the second thin film transistorcomprises a first semiconductor layer, a first gate insulating film, anda first gate electrode; wherein the first semiconductor layer comprisesa first channel formation region, a first impurity region and a secondimpurity region; wherein a concentration of impurity in the firstimpurity region is lower than a concentration of impurity in the secondimpurity region; wherein each of the third thin film transistorscomprises a second semiconductor layer, a second gate insulating filmand a second gate electrode; wherein the second semiconductor layercomprises a second channel formation region, a third impurity region anda fourth impurity region; wherein a concentration of impurity in thethird impurity region is lower than a concentration of impurity in thefourth impurity region; wherein an overlap length of the third impurityregion and the second gate electrode is larger than an overlap length ofthe first impurity region and the first gate electrode; wherein theliquid crystal display device has light emitting diodes used as abacklight; wherein the liquid crystal display device and the lightemitting diodes are driven by a field sequential driving method, andwherein the first thin film transistor and the second thin filmtransistor are connected in series, between a pixel electrode and asource signal line.
 2. A liquid crystal display device according toclaim 1, wherein the first semiconductor layer and the secondsemiconductor layer comprise a crystalline poly-silicon.
 3. A liquidcrystal display device according to claim 1, wherein one of the lightemitting diodes is red, green, or blue.
 4. The liquid crystal displaydevice according to claim 1, a diagonal size of the display is largerthan 10 inches.
 5. A liquid crystal display device comprising: a firstthin film transistor in a pixel; second thin film transistors in asource driver and a gate driver; an optically compensated birefringencemode liquid crystal over the first thin film transistor in the pixel;wherein the first thin film transistor comprises a first semiconductorlayer, a first gate insulating film over the first semiconductor layer,and a first gate electrode over the first gate insulating film; whereinthe first semiconductor layer comprises a first channel formationregion, a first impurity region and a second impurity region; wherein aconcentration of impurity in the first impurity region is lower than aconcentration of impurity in the second impurity region; wherein thesecond thin film transistor comprises a second semiconductor layer, asecond gate insulating film over the second semiconductor layer, and asecond gate electrode over the second gate insulating film; wherein thesecond semiconductor layer comprises a second channel formation region,a third impurity region and a fourth impurity region; wherein aconcentration of impurity in the third impurity region is lower than aconcentration of impurity in the fourth impurity region; wherein thethird impurity region overlaps a part of the second gate electrode;wherein an overlap length of the third impurity region and the secondgate electrode is larger than an overlap length of the first impurityregion and the first gate electrode; wherein the liquid crystal displaydevice has light emitting diodes used as a backlight; and wherein theliquid crystal display device and the light emitting diodes are drivenby a field sequential driving method.
 6. A liquid crystal display deviceaccording to claim 5, wherein the first semiconductor layer and thesecond semiconductor layer comprise a crystalline poly-silicon.
 7. Aliquid crystal display device according to claim 5, wherein one of thelight emitting diodes is red, green, or blue.
 8. The liquid crystaldisplay device according to claim 5, a diagonal size of the display islarger than 10 inches.
 9. A liquid crystal display device comprising: afirst thin film transistor in a pixel; second thin film transistors in asource driver and a gate driver; an optically compensated birefringencemode liquid crystal over the first thin film transistor; and wherein thefirst thin film transistor comprises a first semiconductor layer, afirst gate insulating film over the first semiconductor layer, and firstgate electrode over the first gate insulating film; wherein the firstsemiconductor layer comprises a first channel formation region, a firstimpurity region and a second impurity region; wherein a concentration ofimpurity in the first impurity region is lower than a concentration ofimpurity in the second impurity region; wherein the first impurityregion does not overlap the first gate electrode; wherein the secondthin film transistor comprises a second semiconductor layer, a secondgate insulating film over the second semiconductor layer, and a secondgate electrode over the second gate insulating film; wherein the secondsemiconductor layer comprises a second channel formation region, a thirdimpurity region and a fourth impurity region; wherein a concentration ofimpurity in the third impurity region is lower than a concentration ofimpurity in the fourth impurity region; wherein the third impurityregion overlaps a part of the second gate electrode; wherein an overlaplength of the third impurity region and the second gate electrode islarger than an overlap length of the first impurity region and the firstgate electrode: wherein the liquid crystal display device has lightemitting diodes used as a backlight; wherein the first thin filmtransistor, the source driver and the gate driver are formed over thesame substrate; and wherein the liquid crystal display device and thelight emitting diodes are driven by a field sequential driving method.10. A liquid crystal display device according to claim 9, wherein thefirst semiconductor layer and the second semiconductor layer comprise acrystalline poly-silicon.
 11. A liquid crystal display device accordingto claim 9, wherein one of the light emitting diodes is red, green, orblue.
 12. The liquid crystal display device according to claim 9, adiagonal size of the display is larger than 10 inches.
 13. A liquidcrystal display device comprising: a first thin film transistor and asecond thin film transistor, both of them in a pixel; and third thinfilm transistors in a source driver and a gate driver; an opticallycompensated birefringence mode liquid crystal over the first thin filmtransistor and the second thin film transistor in the pixel; whereineach of the first thin film transistor and the second thin filmtransistor comprises a first semiconductor layer, a first gateinsulating film and a first gate electrode; wherein the firstsemiconductor layer comprises a channel formation region, a firstimpurity region and a second impurity region; wherein a concentration ofimpurity in the first impurity region is lower than a concentration ofimpurity in the second impurity region; wherein each of the third thinfilm transistors comprises a second semiconductor layer, a second gateinsulating film and a second gate electrode; wherein the secondsemiconductor layer comprises a second channel formation region, a thirdimpurity region and a fourth impurity region; wherein a concentration ofimpurity in the third impurity region is lower than a concentration ofimpurity in the fourth impurity region; wherein an overlap length of thethird impurity region and the second gate electrode is larger than anoverlap length of the first impurity region and the first gateelectrode; wherein the first impurity region does not overlap the firstgate electrode; wherein the liquid crystal display device has lightemitting diodes used as a backlight; and wherein the liquid crystaldisplay device and the light emitting diodes are driven by a fieldsequential driving method, and wherein the first thin film transistorand the second thin film transistor are connected in series, between apixel electrode and a source signal line.
 14. A liquid crystal displaydevice according to claim 13, wherein the first semiconductor layer andthe second semiconductor layer comprise a crystalline poly-silicon. 15.A liquid crystal display device according to claim 13, wherein one ofthe light emitting diodes is red, green, or blue.
 16. The liquid crystaldisplay device according to claim 13, a diagonal size of the display islarger than 10 inches.
 17. A liquid crystal display device comprising: afirst thin film transistor in a pixel; second thin film transistors in asource driver and a gate driver; and an optically compensatedbirefringence mode liquid crystal over the first thin film transistor;wherein the first thin film transistor comprises a first semiconductorlayer, a first gate insulating film, and a first gate electrode; whereinthe first semiconductor layer comprises a first channel formationregion, a first impurity region and a second impurity region; wherein aconcentration of impurity in the first impurity region is lower than aconcentration of impurity in the second impurity region; wherein thefirst impurity region does not overlap the first gate electrode; whereineach of the second thin film transistors comprises a secondsemiconductor layer, a second gate insulating film, and a second gateelectrode; wherein the second semiconductor layer comprises a secondchannel formation region, a third impurity region and a fourth impurityregion; wherein a concentration of impurity in the third impurity regionis lower than a concentration of impurity in the fourth impurity region;wherein the third impurity region overlaps a part of the second gateelectrode; and wherein the first thin film transistor, the source driverand the gate driver are formed over the same substrate; wherein anoverlap length of the third impurity region and the second gateelectrode is larger than an overlap length of the first impurity regionand the first gate electrode; wherein the liquid crystal display devicehas light emitting diodes used as a backlight; and wherein the liquidcrystal display device and the light emitting diodes are driven by afield sequential driving method.
 18. A liquid crystal display deviceaccording to claim 17, wherein the first semiconductor layer and thesecond semiconductor layer comprise a crystalline poly-silicon.
 19. Aliquid crystal display device according to claim 17, wherein one of thelight emitting diodes is red, green, or blue.
 20. The liquid crystaldisplay device according claim 17, a diagonal size ofthe display islarger than 10 inches.